The present invention is directed to the field of photolithography. Specifically, the present invention is directed to a novel apparatus for exposing a pattern onto a photoresist-coated substrate cylinder and the process of using the apparatus.
In the art of photolithography, an image contained on a photomask is transferred to a photoresist-coated substrate. The photoresist (also referred to simply as "resist") is sensitive to light of a specific wavelength such that when light of the appropriate wavelength passes through the transparent areas on the photomask and strikes the photoresist, the photoresist is chemically changed. Developing of the exposed photoresist removes portions of the photoresist from the substrate corresponding to the exposed or unexposed portions of the photoresist, depending on the type of photoresist used.
The image thus formed in the resist may be utilized in a number of ways. In certain applications, the resist pattern is used as a mask, protecting the substrate from an etching solution that dissolves the non-coated portion of the substrate. In other applications the resist can serve as a "mold," wherein metal is electroformed into the cavities from which the resist has been removed. In yet other applications the patterned resist is used directly to transfer ink from one surface to another as part of a printing process.
Conventional photolithography utilizes a flat photomask to transfer an image to a flat photoresist-covered substrate, wherein the :flat photomask and flat substrate are positioned in parallel planes. Collimated light perpendicular to the photomask and substrate is passed through the flat photomask to expose areas on the substrate corresponding to transparent areas of the photomask without undercutting the opaque areas of the photomask.
Conventional photolithography is not readily adaptable to curved or cylindrical substrates (also referred to herein as "substrate cylinders"). When a curved or cylindrical substrate is used with a flat photomask, the collimated light striking the substrate cylinder is perpendicular to only a narrow line on the surface of the substrate cylinder. The remainder of the substrate cylinder is exposed to nonperpendicular, or sloping, light. The nonperpendicular light results in decreased resolution of the exposed pattern, due in part to the nonperpendicular rays undercutting the opaque portion of the photomask.
Various attempts have been made to modify conventional photolithography tools and methods to expose the photomask image onto a curved or cylindrical substrate without a decrease in pattern resolution. One such method provides a narrow band of exposure light to prevent exposure of areas of the substrate cylinder removed from the line at which the light is perpendicular to the substrate cylinder. The substrate cylinder is rotated under the flat photomask as the photomask moves horizontally so as to expose the entire circumference of the substrate cylinder. Although a very narrow slit will minimize distortion from sloped light, such slits allow exposure of only a small portion of the circumference of the substrate cylinder at any one point in time, thereby significantly increasing the time required to expose the entire circumference of the substrate cylinder. In addition, it is difficult to match or align the pattern at the beginning and end of the pattern (referred to herein as the 0 and 360 degree positions). Further, it is difficult to maintain the alignment between the narrow band of light and the longitudinal axis of the substrate cylinder and, therefore, some distortion of the image is generated from such operations.
Another known method for exposing a pattern onto photoresist-coated substrate cylinders utilizes a variation of conventional contact photolithography. Contact photolithography involves direct contact between the photomask and the resist-coated substrate, which minimizes the area for exposure light to undercut the photomask and thereby provides high pattern resolution. In conventional contact photolithography, both the photomask and substrate are flat. In the variation of contact photolithography developed for exposing a pattern onto cylinders, the photomask is formed as a cylindrical sleeve having an inside diameter slightly larger than the outside diameter of the resist-coated substrate cylinder. This arrangement leaves small gaps between the photomask sleeve and substrate cylinder, which allows some undercutting of the photomask by the exposure light, thereby reducing pattern resolution. Additionally, photomask sleeves are problematic in that the soft-baked resist that is coated onto the substrate cylinder generally remains somewhat tacky, making it difficult to slide a close-fitting photomask sleeve over the substrate cylinder. It is preferable to fabricate such a photomask sleeve with the photomask image on the interior of the sleeve, so that the photomask image will be in contact with the exterior of the cylindrical substrate to minimize undercutting of the photomask. However, it is difficult to fabricate a photomask sleeve wherein the photomask image is on the interior surface of the sleeve.
Conventional photolithography is particularly unsuited to exposing patterns onto substrate cylinders of a very small diameter, such as those used to produce cardiovascular stents, because the results of undercutting are especially evident in substrate cylinders of a small diameter. The prior attempts to adapt photolithography to cylindrical substrates are not readily adaptable to substrate cylinders of a very small diameter. As a result, non-photolithographic processes for forming patterns on small substrate cylinders have also been attempted. In one such process, a resist pattern is formed on a substrate cylinder using a computer-controlled machine and laser to remove the unwanted resist. This process also is problematic in that the laser leaves rough edges that cannot be tolerated in many applications, for example, in cardiovascular stents.
Such difficulties are discussed more fully in co-pending U.S. Pat. application Ser. No. 08.backslash.819,757 now U.S. Pat. No. 6,019,784 for Electroformed Stents and co-pending U.S. Pat. application Ser. No. 09.backslash.201,972 for Uniform which applications were filed by the inventor of the present application. In these applications, the inventor discusses the benefits of using photolithography to from a resist pattern on a mandrel used for electroforming coronary stents. Coronary stents have a small diameter, generally about 0.06 inches and must have smooth edges so as not to damage the interior of the artery, or other vessel, into which they are inserted. Further, the inventor's preferred stent pattern involves an intricate series of loops and bands. Such small and/or intricate pattern should be produced with a high resolution.
Thus, while procedures for exposing a pattern in a resist-coated substrate cylinder are known, a need remains for an efficient method for exposing a continuous, high-resolution pattern around the entire circumference of a photoresist-coated substrate cylinder, including a substrate cylinder of a very small diameter.